1. Field of the Invention
The invention relates to methods and designs for obtaining improved water distribution within the cells of a fuel cell series stack during shutdown and, more particularly, to the shutdown of solid polymer electrolyte fuel cell stacks.
2. Description of the Related Art
Fuel cell systems are presently being developed for use as power supplies in a wide variety of applications, such as stationary power plants and portable power units. Such systems offer the promise of economically delivering power while providing environmental benefits.
Fuel cells convert fuel and oxidant reactants to generate electric power and reaction products. They generally employ an electrolyte disposed between cathode and anode electrodes. A catalyst typically induces the desired electrochemical reactions at the electrodes.
A preferred fuel cell type, particularly for portable and motive applications, is the solid polymer electrolyte (SPE) fuel cell, which comprises a solid polymer electrolyte membrane and operates at relatively low temperatures.
SPE fuel cells employ a membrane electrode assembly (MEA) that comprises the solid polymer electrolyte or ion-exchange membrane disposed between the cathode and anode. Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte membrane. The catalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support). The catalyst layers may contain an ionomer similar to that used for the solid polymer electrolyte membrane (e.g., Nafion®). The electrodes may also contain a porous, electrically conductive substrate that may be employed for purposes of mechanical support, electrical conduction, and/or reactant distribution, thus serving as a fluid diffusion layer. Flow field plates for directing the reactants across one surface of each electrode or electrode substrate, are disposed on each side of the MEA. In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell series stack.
During normal operation of a SPE fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant at the cathode catalyst to generate water reaction product.
In some fuel cell applications, the demand for power can essentially be continuous and thus the stack may rarely be shutdown (such as for maintenance). However, in many applications (e.g., automotive), a fuel cell stack may frequently be stopped and restarted with significant storage periods in between. Such cyclic use can pose certain problems in SPE fuel cell stacks. For instance, in U.S. Patent Application Publication Nos. US 2002/0076582 and US 2002/0076583, it is disclosed how conditions leading to cathode corrosion can arise during startup and shutdown and that corrosion may be reduced by rapidly purging the anode flow field with an appropriate fluid.
Other problems that can arise from cyclic use relate to the water content remaining and its distribution in the stack after shutdown. For instance, liquid water accumulations in the stack can result from too much water remaining and/or undesirable distribution during shutdown. Such accumulations of liquid water can adversely affect cell performance by blocking the flow of reactants and/or by-products. Perhaps even worse, if the fuel cell stack is stored at below freezing temperatures, liquid water accumulations in the cells can freeze and possibly result in permanent damage to the cells. On the other hand, with too little water remaining, the conductivity of the membrane electrolyte can be substantially reduced, with resulting poor power capability from the stack when restarting.
PCT application No. US2004/029905 filed Sep. 10, 2004 by the same applicant discloses shutdown methods and stack designs that allow for a desirable distribution of liquid water within the stack upon shutdown and thereby provide for improved freeze start performance. Appropriate thermal gradients are employed during shutdown to achieve the desirable liquid water distribution. In a related manner, PCT application WO2004/107839 also discloses methods of maintaining fuel cell performance despite freeze/thaw cycles by employing a thermal gradient to cause appropriate water migration. Therein, the methods establish a gradient in which the fuel cell cathode is hotter than the anode.
The present invention relates to improved methods and designs for achieving a desirable thermal gradient and hence liquid water distribution upon shutdown of a stack.